Valve timing control system of internal combustion engine

ABSTRACT

In a valve timing control system of an internal combustion engine employing both an electric-motor-driven intake valve timing control device (VTC) and an electric-motor-driven exhaust VTC having similar fundamental configurations, a maximum drive-efficiency motor-speed range of a first electric motor of the intake VTC and a maximum drive-efficiency motor-speed range of a second electric motor of the exhaust VTC are set to differ from each other depending on two different working ranges respectively suitable for the intake VTC and the exhaust VTC such that the maximum drive-efficiency motor-speed range of the intake VTC is set higher than that of the exhaust VTC. A reduction ratio of a first speed reducer configured to reduce a rotational speed of the first electric motor is set higher than that of a second speed reducer configured to reduce a rotational speed of the second electric motor.

TECHNICAL FIELD

The present invention relates to a valve timing control system of an internal combustion engine for variably controlling valve timings (i.e., valve open timing and valve closure timing) of intake and exhaust valves.

BACKGROUND ART

A valve timing control system, which is configured to change an angular phase of a camshaft relative to a timing sprocket by virtue of hydraulic pressure, is generally known. In recent years, there have been proposed and developed various valve timing control systems in which an angular phase of a camshaft relative to a timing sprocket that is configured to rotate in synchronism with rotation of an engine crankshaft is changed by transmitting rotary motion (torque) of an electric motor through a speed reducer to the camshaft, so as to variably control intake-valve timing and exhaust-valve timing.

One such valve timing control system has been disclosed in Japanese Unexamined Patent Application Publication No. 2006-207398 (hereinafter is referred to as “JP2006-207398”), corresponding to U.S. Pat. No. 7,603,223, issued on Oct. 13, 2009. In the valve timing control system disclosed in JP2006-207398, two electric-motor-driven valve timing control devices are mounted respectively on the intake camshaft and the exhaust camshaft.

SUMMARY OF THE INVENTION

By the way, the working range of the intake valve timing control (VTC) device and the working range of the exhaust valve timing control (VTC) device generally differ from each other, depending on engine operating conditions. Regarding the intake electric-motor-driven VTC device, its electric motor tends to be frequently driven over the entire engine operating range after the engine has been started. In particular, under high engine-speed conditions, the drive load on the motor of the intake VTC device tends to increase. In contrast, regarding the exhaust electric-motor-driven VTC device, its electric motor tends to be frequently driven at middle engine speeds. Thus, in such a middle engine-speed range, the drive load on the motor of the exhaust VTC device tends to increase.

However, in the case of the valve timing control system disclosed in JP2006-207398, the electric motors, respectively applied to the intake VTC device and the exhaust VTC device, have almost the same configuration. Additionally, the speed reducers are the same in the intake VTC device and the exhaust VTC device. For the reasons discussed above, assume that the drive efficiencies of the electric motors of the intake VTC device and the exhaust VTC device are both set to a drive efficiency which is predetermined to be suitable for the drive load in a specific engine-speed range within which the electric motor of one of the intake VTC device and the exhaust VTC device frequently operates. In such a case, there is an increased tendency for the drive efficiency of the electric motor of the other VTC device, which is unsuitable for the specific engine-speed range, to be undesirably decreased.

Accordingly, it is an object of the invention to provide a valve timing control system of an internal combustion engine, configured to efficiently drive each of electric motors of an intake valve timing control device and an exhaust valve timing control device.

In order to accomplish the aforementioned and other objects of the present invention, a valve timing control system of an internal combustion engine comprises an electric-motor-driven intake valve timing control device installed on an intake camshaft and having a first electric motor provided to generate torque by energizing the first electric motor, and an electric-motor-driven exhaust valve timing control device installed on an exhaust camshaft and having a second electric motor provided to generate torque by energizing the second electric motor, wherein a maximum drive-efficiency motor-speed range of the first electric motor of the electric-motor-driven intake valve timing control device and a maximum drive-efficiency motor-speed range of the second electric motor of the electric-motor-driven exhaust valve timing control device are set to differ from each other depending on a first working range suitable for the electric-motor-driven intake valve timing control device and a second working range suitable for the electric-motor-driven exhaust valve timing control device.

According to another aspect of the invention, a valve timing control system of an internal combustion engine comprises an electric-motor-driven intake valve timing control device installed on an intake camshaft, the electric-motor-driven intake valve timing control device comprising a first electric motor provided to generate torque by energizing the first electric motor, and a first speed reducer configured to reduce a rotational speed of the first electric motor, and transmit the reduced rotational speed to the intake camshaft for changing intake valve timing, and an electric-motor-driven exhaust valve timing control device installed on an exhaust camshaft, the electric-motor-driven exhaust valve timing control device comprising a second electric motor provided to generate torque by energizing the second electric motor, and a second speed reducer configured to reduce a rotational speed of the second electric motor, and transmit the reduced rotational speed to the exhaust camshaft for changing exhaust valve timing, wherein a reduction ratio of the first speed reducer for a reduction in the rotational speed of the first electric motor and a reduction ratio of the second speed reducer for a reduction in the rotational speed of the second electric motor are set to differ from each other.

According to a further aspect of the invention, a valve timing control system of an internal combustion engine, comprises an electric-actuator equipped intake valve timing control device installed on an intake camshaft, the electric-actuator equipped intake valve timing control device comprising a first electric actuator provided to generate torque by energizing the first electric actuator, and a first speed reducer configured to reduce a rotational speed of the first electric actuator, and transmit the reduced rotational speed to the intake camshaft for changing intake valve timing, and an electric-actuator equipped exhaust valve timing control device installed on an exhaust camshaft, the electric-actuator equipped exhaust valve timing control device comprising a second electric actuator provided to generate torque by energizing the second electric actuator, and a second speed reducer configured to reduce a rotational speed of the second electric actuator, and transmit the reduced rotational speed to the exhaust camshaft for changing exhaust valve timing, wherein a reduction ratio of the first speed reducer for a reduction in the rotational speed of the first electric actuator and a reduction ratio of the second speed reducer for a reduction in the rotational speed of the second electric actuator are set to differ from each other.

The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plane cross-sectional view illustrating the essential part of the embodiment of a valve timing control system.

FIG. 2 is a view taken in the direction of the arrow A of FIG. 1.

FIG. 3 is a cross-sectional view taken along the line B-B of FIG. 2 and illustrating the longitudinal cross section of an intake valve timing control (VTC) device of the embodiment.

FIG. 4 is a perspective disassembled view illustrating major component parts constructing the VTC device of the embodiment.

FIG. 5 is a lateral cross section taken along the line C-C of FIG. 3.

FIG. 6 is a lateral cross section taken along the line D-D of FIG. 3.

FIG. 7 is a lateral cross section taken along the line E-E of FIG. 3.

FIG. 8 is a cross-sectional view taken along the line F-F of FIG. 2 and illustrating the longitudinal cross section of an exhaust VTC device of the embodiment.

FIG. 9 is a lateral cross section taken along the line G-G of FIG. 8.

FIG. 10 is a lateral cross section taken along the line H-H of FIG. 8.

FIG. 11 is a characteristic diagram illustrating the relationship between a motor speed of each of the intake-VTC-side electric motor and the exhaust-VTC-side electric motor and a drive efficiency of each of the electric motors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, particularly to FIGS. 1-2, the valve timing control system of the embodiment includes an intake camshaft 02 rotatably supported on the upper deck of a cylinder head 01 through a frame-shape camshaft-journal bearing member 06 fixedly connected onto the upper deck of cylinder head 01, an exhaust camshaft 03 rotatably supported on the cylinder head 01 through the camshaft-journal bearing member 06 and arranged parallel to the intake camshaft 02, an electric-motor-driven intake valve timing control device (hereinafter referred to as “intake VTC”) 04 installed on the front end of intake camshaft 02, and an electric-motor-driven exhaust valve timing control device (hereinafter referred to as “exhaust VTC”) 05 installed on the front end of exhaust camshaft 03.

Camshaft-journal bearing member 06 is made from aluminum alloy. Both ends of intake camshaft 02, and the intermediate sections of intake camshaft 02 are rotatably supported by means of bearings, that is, rotatably sandwiched between half-round sections formed on the upper deck of cylinder head 01 and upper bearing halves of camshaft-journal bearing member 06 fit into the respective half-round sections of cylinder head 01. In a similar manner, both ends of exhaust camshaft 03, and the intermediate sections of exhaust camshaft 03 are rotatably supported by means of bearings, that is, rotatably sandwiched between half-round sections formed on the upper deck of cylinder head 01 and upper bearing halves of camshaft-journal bearing member 06 fit into the respective half-round sections of cylinder head 01. The front end of camshaft-journal bearing member 06 is formed integral with a chain cover 07 configured to partially cover both the intake VTC 04 and the exhaust VTC 05. A VTC cover 3 is bolted to the front end of chain cover 07 on the side of intake VTC 04 for hermetically covering the front end of intake VTC 04. In a similar manner, a VTC cover 3 is bolted to the front end of chain cover 07 on the side of exhaust VTC 05 for hermetically covering the front end of exhaust VTC 05.

[Intake VTC]

First, the above-mentioned intake VTC 04 is hereunder explained in detail. As shown in FIGS. 3-4, intake VTC 04 is comprised of a sprocket 1 (serving as a driving rotary member) that rotates in synchronism with rotation of an engine crankshaft, and a phase change mechanism (a phase converter) 2 installed between the sprocket 1 and the intake camshaft 02 for changing a relative angular phase between the sprocket 1 and the intake camshaft 02 depending on an engine operating condition.

Sprocket 1 is comprised of an annular sprocket body 1 a, a timing gear 1 b formed integral with the outer periphery of sprocket body 1 a, and an internal-tooth structural member 19. Sprocket body 1 a is made from iron-based metal material, and formed with a stepped inner peripheral portion and formed integral with the timing gear 1 b. Timing gear 1 b receives torque from the crankshaft through a timing chain (not shown) wound on both a sprocket on the crankshaft and the sprocket 1 on the intake camshaft side. Internal-tooth structural member 19 is formed integral with the front end of sprocket body 1 a on the intake camshaft side.

Also, sprocket 1 is rotatably supported by a large-diameter ball bearing 43 interleaved between the sprocket body 1 a and a driven rotary member, simply, a driven member 9 (described later) fixedly connected to the front end of intake camshaft 02, so as to permit rotary motion of intake camshaft 02 relative to sprocket 1.

Large-diameter ball bearing 43 is comprised of an outer ring 43 a, an inner ring 43 b, and balls 43 c confined between outer and inner rings 43 a-43 b. The outer ring 43 a is fixed to the inner periphery of sprocket body 1 a, whereas the inner ring 43 b is fixed to the outer periphery of driven member 9 (described later).

Sprocket body 1 a has an outer-ring retaining annular groove 60 formed and cut in its inner peripheral surface. Outer-ring retaining annular groove 60 is formed as a shouldered annular groove into which the outer ring 43 a of large-diameter ball bearing 43 is axially press-fitted. The shouldered portion of outer-ring retaining annular groove 60 serves to position one axial end face (i.e., a forward end face, viewing FIG. 3) of the outer ring 43 a in place.

Internal-tooth structural member 19 is formed integral with the circumference of the front end of sprocket body 1 a, and formed into a cylindrical shape extended toward an electric motor 12 (described later) of phase converter 2. Internal-tooth structural member 19 is formed on its inner periphery with a plurality of waveform internal teeth 19 a. The annular rear end face of an annular female screw-threaded member 6, formed integral with a housing 5 (described later), and the annular front end face of internal-tooth structural member 19 are arranged to be axially opposed to each other.

An annular retainer plate 61 is located at the rear end of sprocket body 1 a, facing apart from the internal-tooth structural member 19. Retainer plate 61 is made from a metal plate. As shown in FIG. 3, the outside diameter of retainer plate 61 is dimensioned to be approximately equal to that of the sprocket body 1 a. The inside diameter of retainer plate 61 is set or dimensioned to be less than the inside diameter of the outer ring 43 a of ball bearing 43 and also dimensioned to be approximately equal to the outside diameter of the inner ring 43 b of ball bearing 43.

Hence, the inner peripheral portion 61 a (see FIG. 4) of retainer plate 61 is arranged to be axially opposed to the rearward end face 43 e of the outer ring 43 a of ball bearing 43 with a given clearance space in such a manner as to cover the rearward end face 43 e of the outer ring 43 a. Also, the inner peripheral portion 61 a of annular retainer plate 61 has a radially-inward protruding stopper 61 b integrally formed at a given circumferential angular position of the inner peripheral portion 61 a.

As seen in FIG. 6, the radially-inward protruding stopper 61 b is formed into a substantially sector. The innermost edge 61 c of stopper 61 b is configured to be substantially conformable to a shape of the circular-arc peripheral surface of a stopper groove 02 b (described later) of the front end of intake camshaft 02. The outer peripheral portion of retainer plate 61 is formed with circumferentially equidistant-spaced, six bolt insertion holes 61 d (through holes) through which bolts 7 are inserted.

Furthermore, an annular pressing member 62 is interleaved between the inside face (the left-hand side face) of retainer plate 61 and the rearward end face 43 e of the outer ring 43 a of ball bearing 43. Pressing member 62 is provided for applying a slight push from the inside face of retainer plate 61 to the rearward end face 43 e of the outer ring 43 a, when the annular female screw-threaded member 6 (housing 5), the sprocket 1, and the retainer plate 61 are integrally connected to each other by fastening them together with bolts 7.

In a similar manner to the six bolt insertion holes 61 d (through holes) formed in the retainer plate 61, the outer peripheral portion of sprocket body 1 a (internal-tooth structural member 19) is formed with circumferentially equidistant-spaced, six bolt insertion holes 1 c (through holes). On the other hand, the annular female screw-threaded member 6 is formed with six female screw threads 6 a configured to be conformable to respective circumferential positions of bolt insertion holes 1 c (bolt insertion holes 61 d). Hence, the annular female screw-threaded member 6 (the housing 5), the sprocket 1, and the retainer plate 61 are integrally connected to each other by axially fastening them together with bolts 7.

Outside diameters of the sprocket body 1 a, the internal-tooth structural member 19, the retainer plate 61, and the female screw-threaded member 6 are dimensioned to be almost the same.

As shown in FIGS. 1-3, chain cover 07 is laid out and bolted to an engine body in a manner so as to vertically extend for covering the timing chain (not shown) wound on the sprocket. Chain cover 07 has a substantially circular opening 07 a (see FIGS. 1 and 3) configured to be conformable to the contour of intake VTC 04 and a substantially circular opening 07 b (see FIG. 1) configured to be conformable to the contour of exhaust VTC 05. The openings 07 a-07 b are formed in respective annular walls of the front end of chain cover 07. Each of the annular walls has four boss sections 07 c integrally formed on the inner periphery of the annular wall and circumferentially spaced from each other. Four female screw-threads 07 d are machined in respective boss sections 07 c such that female screw-threads 07 d extend from the front end face of the annular wall into the respective boss sections.

As shown in FIGS. 1 and 3, VTC cover 3 on the side of intake VTC 04 is made from aluminum alloy and formed into a substantially cup shape. The VTC cover 3 is comprised of a cup-shaped cover main body 3 a and an annular flange 3 b formed integral with the circumference of the right-hand side opening end (viewing FIG. 1) of cover main body 3 a. Cover main body 3 a is configured to cover the front end of phase converter 2. Cover main body 3 a has a slightly axially-extending cylindrical wall portion 3 c integrally formed at a given position deviated radially upward from the center of the frontal flat wall portion of cover main body 3 a. The cylindrical wall portion 3 c has a retaining through-hole 3 d formed therein.

Annular flange 3 b is integrally formed with four tab-like portions 3 e, circumferentially spaced apart from each other at intervals of approximately 90 degrees. Four bolt insertion holes 3 f (through holes) are bored in respective tab-like portions 3 e of the annular flange 3 b. VTC cover 3 is fixedly connected to the chain cover 07 by means of bolts 54 (see FIG. 3), which are inserted through the respective bolt insertion holes 3 f and screwed into the female screw-threads 07 d formed in the respective boss sections 07 c of chain cover 07.

Also, the inner periphery of the right-hand side opening end (viewing FIG. 3) of cover main body 3 a is formed as a shouldered oil-seal retaining annular groove 3 h. A large-diameter oil seal 50 is interleaved between the shouldered oil-seal retaining annular groove 3 h of cover main body 3 a and the outer peripheral surface of housing 5. Large-diameter oil seal 50 is formed into a substantially C-shape in lateral cross section. Oil seal 50 is made from synthetic rubber (a base material), and also a core metal is buried in the base material. The cylindrical outer peripheral surface of oil seal 50 is fitted to the shouldered oil-seal retaining annular groove 3 h of cover main body 3 a of VTC cover 3 in a fluid-tight fashion, whereas the inner periphery of oil seal 50 (that is, a spring-loaded single lip and a non-spring-loaded dust lip) is fitted onto the outer periphery of housing 5 in a fluid-tight fashion.

As shown in FIGS. 3-4, housing 5 is comprised of a housing main body 5 a made from iron-based metal material and formed into a substantially cylindrical shape with a rear end face (a bottom face) by pressing, and a seal plate 11 made from synthetic resin (non-magnetic material) and provided for sealing the axially forward opening (the left-hand side opening end, viewing FIG. 3) of housing main body 5 a.

Housing main body 5 a has a bottom 5 b formed at its rear end, facing the internal-tooth structural member 19. Housing main body 5 a is formed in a substantially center of the bottom 5 b with a large-diameter eccentric-shaft insertion hole into which an eccentric shaft 39 (described later) is inserted. An axially-leftward extending cylindrical portion 5 c is formed integral with the annular edge of the eccentric-shaft insertion hole in a manner so as to somewhat extend in the axial direction of intake camshaft 02. The previously-discussed annular female screw-threaded member 6 is formed integral with the outer periphery of the bottom 5 b of housing 5.

Intake camshaft 02 has two rotary drive cams (per cylinder) integrally formed on its outer periphery for operating the associated two intake valves (not shown) per one engine cylinder. Also, intake camshaft 02 has a flanged portion 02 a integrally formed at its front end. Driven member 9 is fixedly connected to the front end of the flanged portion 02 a by means of a cam bolt 10 under a condition where the front end face of the flanged portion 02 a has been kept in abutted-engagement with the rear end face of a fixed-end portion 9 a of driven member 9. As seen in FIG. 3, the outside diameter of flanged portion 02 a is dimensioned to be slightly greater than that of the fixed-end portion 9 a of driven member 9. Hence, after installation of all component parts, the circumference of the front end face of the flanged portion 02 a of intake camshaft 02 is brought into abutted-engagement with the rearward end face of the inner ring 43 b of large-diameter ball bearing 43.

As shown in FIG. 6, the outer periphery of the flanged portion 02 a of intake camshaft 02 is partially machined or cut as the stopper groove 02 b recessed along the circumferential direction. The radially-inward protruding stopper 61 b of retainer plate 61 is circumferentially moveably installed in the stopper groove 02 b. Stopper groove 02 b is formed into a circular-arc shape having a specified circumferential length to permit a circumferential movement of stopper 61 b within a limited motion range determined based on the specified circumferential length. Hence, a maximum phase-advance position of intake camshaft 02 relative to sprocket 1 is restricted by abutment between the counterclockwise edge of stopper 61 b and the clockwise edge 02 c of stopper groove 02 b. On the other hand, a maximum phase-retard position of intake camshaft 02 relative to sprocket 1 is restricted by abutment between the clockwise edge of stopper 61 b and the counterclockwise edge 02 d of stopper groove 02 b.

As appreciated from the longitudinal cross section of FIG. 3, stopper 61 b is kept in a spaced, contact-free relationship with the fixed-end portion 9 a of driven member 9 in the axial direction, thus adequately suppressing undesirable interference between the stopper 61 b and the fixed-end portion 9 a.

As appreciated from the longitudinal cross section of FIG. 3, cam bolt 10 is comprised of a head 10 a and a shank 10 b formed integral with each other, and an annular washer provided at the boundary of head 10 a and shank 10 b. Shank 10 b is formed on its outer periphery with a male-screw-threaded portion, which is screwed into a female-screw-threaded portion machined into the front end of intake camshaft 02 along the axis of intake camshaft 02.

Driven member 9 is made from iron-based metal material. As seen from the longitudinal cross section of FIG. 3, the driven member 9 is comprised of the disk-shaped fixed-end portion 9 a, an axially-forward-extending cylindrical portion 9 b formed integral with the front end face of disk-shaped fixed-end portion 9 a, and a substantially cylindrical cage 41, which cage is formed integral with the outer periphery of disk-shaped fixed-end portion 9 a and configured to serve as a roller holder for holding a plurality of rollers 48 (rolling elements).

The rear end face of disk-shaped fixed-end portion 9 a is arranged to abut with the front end face of the flanged portion 02 a of intake camshaft 02, and fixedly connected to the flanged portion 02 a by an axial force of cam bolt 10.

As shown in FIG. 3, cylindrical portion 9 b is formed with a central bore 9 d into which the shank 10 b of cam bolt 10 is inserted. A needle bearing 38 is mounted on the outer periphery of cylindrical portion 9 b.

As shown in FIGS. 3-5, cage 41 (the roller holder) is configured to further extend from the outer periphery of disk-shaped fixed-end portion 9 a, and bent into a substantially L shape in longitudinal cross section and formed into a substantially cylindrical shape extending in the same axial direction as the cylindrical portion 9 b and having an annular bottom axially opposed to one sidewall of a ball-bearing outer ring 47 b (described later). More concretely, the substantially cylindrical portion 41 a of cage 41 is configured to extend toward the bottom 5 b of housing 5 through an annular internal space 44 defined between the annular female screw-threaded member 6 and the axially-leftward extending cylindrical portion 5 c. Also, the substantially cylindrical portion 41 a of cage 41 has a plurality of axially-protruding lugs. As a whole, the axially-protruding lugs are shaped into a substantially comb-tooth shape. That is, by virtue of the axially-protruding lugs, each having a substantially rectangular cross-section, a plurality of roller-holding holes 41 b are configured to be equidistant-spaced from each other with a given circumferential interval in the circumferential direction of the outer periphery of disk-shaped fixed-end portion 9 a. Rollers 48 are rotatably held or installed in respective roller-holding holes 41 b. The substantially cylindrical portion 41 a of cage 41 has one fewer roller-holding holes (in other words, one fewer rollers or one fewer axially-protruding lugs) than the number of internal teeth 19 a of internal-tooth structural member 19.

An inner-ring retaining annular groove 63 is machined and defined between the outer periphery of disk-shaped fixed-end portion 9 a and the annular bottom of cage 41 formed integral with each other, for retaining the inner ring 43 b of large-diameter ball bearing 43.

Inner-ring retaining annular groove 63 is formed as a shouldered annular groove configured to be radially opposed to the outer-ring retaining annular groove 60 of sprocket body 1 a. Inner-ring retaining annular groove 63 is comprised of a cylindrical outer peripheral surface extending in the axial direction of intake camshaft 02 and a radially-extending shouldered annular surface configured to extend radially outward from the innermost end of the cylindrical outer peripheral surface. When assembling, the inner ring 43 b of ball bearing 43 is axially press-fitted onto the cylindrical outer peripheral surface. At the same time, the forward end face of the press-fitted inner ring 43 b is brought into abutted-engagement with the shouldered annular surface of inner-ring retaining annular groove 63, to position one axial end face (the forward end face) of the inner ring 43 b in place.

Phase converter 2 is mainly constructed by the electric motor 12 coaxially located at the front end of intake camshaft 02, and a roller speed reducer 8 provided for reducing the rotational speed of the motor output shaft 13 of electric motor 12 and for transmitting the reduced motor speed (in other words, the increased motor torque) to the intake camshaft 02.

As seen in FIGS. 3-4, electric motor 12 is a brush-equipped direct-current (DC) motor. Electric motor 12 is comprised of the housing 5 serving as a yoke and rotating together with the sprocket 1, the motor output shaft 13 rotatably installed in the housing 5, a pair of substantially semi-circular permanent magnets 14-15 fixedly connected onto the inner peripheral surface of housing 5, and a stator 16 fixed to the seal plate 11.

Motor output shaft 13 is formed into a shouldered cylindrical-hollow shape, and serves as an armature. Motor output shaft 13 is constructed by a large-diameter portion 13 a of the intake-camshaft side and a small-diameter portion 13 b of the brush-holder side through a shouldered portion 13 c formed substantially at a midpoint of the axially-extending cylindrical-hollow motor output shaft. An iron-core rotor 17, having a plurality of magnetic poles, is fixedly connected onto the outer periphery of large-diameter portion 13 a. Eccentric shaft 39 is axially press-fitted into the large-diameter portion 13 a, in a manner so as to be axially positioned in place by the inside annular face of shouldered portion 13 c.

An annular member 20 is press-fitted onto the outer periphery of small-diameter portion 13 b. A commutator 21 is axially press-fitted onto the outer peripheral surface of annular member 20, in a manner so as to be axially positioned in place by the outside annular face of shouldered portion 13 c.

Furthermore, a plug 53 is fixed or press-fitted to the inner peripheral surface of small-diameter portion 13 b, for preventing or adequately suppressing undesirable leakage of lubricating oil, which oil is supplied into the cylindrical-hollow motor output shaft 13 and eccentric shaft 39 for lubrication of a ball bearing 37 (described later) as well as the previously-discussed needle bearing 38, to the outside.

Iron-core rotor 17 is formed by a magnetic material having a plurality of magnetic poles. The outer periphery of iron-core rotor 17 is constructed as a bobbin having slots on which coil windings of an electromagnetic coil 18 is wound.

On the other hand, commutator 21 is formed as a substantially annular shape and made from a conductive material. Commutator 21 is divided into a plurality of segments whose number is equal to the number of magnetic poles of iron-core rotor 17. Terminals of the coil winding (not shown) drawn out from electromagnetic coil 18 are electrically connected to each of segments of commutator 21. That is, the terminals of the coil winding are sandwiched and electrically connected to the hemmed section formed on the periphery of commutator 21.

As a whole, the substantially semi-circular permanent magnets 14-15 are formed into a cylindrical shape, and have a plurality of magnetic poles in the circumferential direction. The axial position of each of permanent magnets 14-15 is offset forward from the fixed position of iron-core rotor 17.

As shown in FIG. 7, stator 16 is mainly comprised of a disk-shaped synthetic-resin plate 22, a pair of synthetic-resin brush holders 23 a-23 b, a pair of first brushes 25 a-25 b, a radially-inside electricity-feeding slip ring 26 a, a radially-outside electricity-feeding slip ring 26 b, and pig-tale harnesses 27 a-27 b. Disk-shaped synthetic-resin plate 22 is integrally connected to the inner periphery of seal plate 11. Brush holders 23 a-23 b are attached onto the inside face of synthetic-resin plate 22. The first brushes 25 a-25 b serve as current-supply switching brushes and supported by respective holders 23 a-23 b so as to be radially slidable. The radially-inward ends of first brushes 25 a-25 b are kept in sliding-contact (elastic-contact or electric-contact) with the outer peripheral surface of commutator 21 by respective spring forces of coil springs 24 a-24 b. The radially-inside electricity-feeding slip ring 26 a and the radially-outside electricity-feeding slip ring 26 b are attached to the synthetic-resin plate 22, such that the outside face (the left-hand side face, viewing FIG. 3) of each of electricity-feeding slip rings 26 a-26 b is partially exposed and that the inside face (the right-hand side face, viewing FIG. 3) of each of slip rings 26 a-26 b is buried in the front end face of synthetic-resin plate 22. The first brush 25 a and the electricity-feeding slip ring 26 b are electrically connected to each other via the pig-tale harness 27 a, whereas the first brush 25 b and the electricity-feeding slip ring 26 a are electrically connected to each other via the pig-tale harness 27 b. The radially-inside annular slip ring 26 a and the radially-outside annular slip ring 26 b are laid out to be coaxial with each other with a given aperture.

The previously-discussed seal plate 11 is fitted into an annular groove cut in the inner periphery of the front end of the cylindrical housing main body 5 a of housing 5, and fixedly connected to the front end of housing main body 5 a in place by caulking. Also, the subassembly (11, 22) of seal plate 11 and disk-shaped synthetic-resin plate 22 is formed in its center with a shaft insertion hole 11 a into which one axial end (the left-hand axial end, viewing FIG. 3) of motor output shaft 13 is partially inserted.

Synthetic-resin plate 22 and synthetic-resin brush holders 23 a-23 b are made from heat-resistant synthetic-resin materials. For instance, a PPS (polyphenylene sulfide) synthetic resin material is used as a heat-resistant synthetic resin material.

A brush retainer 28, which is integrally molded of a synthetic resin material (serving as an electrical insulating material), is fixedly connected to the cover main body 3 a. In a similar manner to the synthetic-resin plate 22 and synthetic-resin brush holders 23 a-23 b, brush retainer 28 is also made from a heat-resistant synthetic-resin material, such as a PPS (polyphenylene sulfide) synthetic resin material. As shown in FIGS. 3-4, brush retainer 28 is formed into a substantially L shape in side view. Brush retainer 28 is comprised of a substantially cylindrical brush-retaining portion 28 a, a connector portion 28 b, a pair of laterally-extending tab-like brackets 28 c, 28 c (see FIG. 4), and a pair of terminal strips 31, 31. Brush-retaining portion 28 a is inserted into the retaining through-hole 3 d. Connector portion 28 b is formed integral with the upper end of brush-retaining portion 28 a. Tab-like brackets 28 c, 28 c are formed integral with both sides of brush-retaining portion 28 a. Most of terminal strips 31, 31 are buried in the synthetic-resin brush retainer 28.

Terminal strips 31, 31 are arranged parallel with each other and partly cranked. One end (the downward terminal 31 a) of each of the crank-shaped terminal strips 31 is exposed to the bottom of brush-retaining portion 28 a. The other end (the upward terminal 31 b) of each of terminal strips 31 is configured to protrude into a female fitting groove 28 d of connector portion 28 b. The upward terminals 31 b, 31 b of the two parallel terminal strips 31, 31 are electrically connected to a control unit (not shown) via a male socket (not shown) fitted to the female fitting groove 28 d.

Brush-retaining portion 28 a is configured to extend horizontally (axially). An upper hollow sleeve 29 b is press-fitted into an upper cylindrical-hollow through hole bored in the brush-retaining portion 28 a. In a similar manner, a lower hollow sleeve 29 a is press-fitted into a lower cylindrical-hollow through hole bored in the brush-retaining portion 28 a. A pair of second brushes 30 a, 30 a are supported by the respective hollow sleeves so as to be axially slidable. The tips of second brushes 30 a, 30 a are kept in sliding-contact (abutted-engagement or electric-contact) with respective slip rings 26 a and 26 b.

For instance, as a preferable material for each of hollow sleeves 29 a-29 b, brass such as 70/30 brass (JIS alloy C2600) is used, thereby ensuring a favorable sliding performance of each of second brushes 30 a, 30 a.

Each of second brushes 30 a, 30 a is formed into a substantially rectangular parallelopiped shape. A second coil spring 32 a is disposed between the downward terminal exposed to the bottom of the upper cylindrical-hollow through hole of brush-retaining portion 28 a and the associated second brush 30 a under preload. In a similar manner, a second coil spring 32 a is disposed between the downward terminal exposed to the bottom of the lower cylindrical-hollow through hole of brush-retaining portion 28 a and the associated second brush 30 a under preload. Thus, the tips of second brushes 30 a, 30 a are permanently forced or biased toward respective slip rings 26 a and 26 b by the spring forces of second coil springs 32 a, 32 a, so as to bring the tips of second brushes 30 a, 30 a into elastic-contact with the respective outside faces of slip rings 26 a-26 b.

Additionally, a flexible pig-tale harness 33 is connected between the square base of second brush 30 a and the downward terminal 31 a exposed to the bottom of the upper cylindrical-hollow through hole of brush-retaining portion 28 a by welding, to provide electric connection. In a similar manner, a flexible pig-tale harness 33 is electrically connected between the square base of second brush 30 a and the downward terminal 31 a exposed to the bottom of the lower cylindrical-hollow through hole of brush-retaining portion 28 a by welding, to provide electric connection. The lengths of pig-tale harnesses 33, 33 are set to appropriate lengths sufficient to restrict maximum sliding movements (maximum axially-extended positions) of second brushes 30 a, 30 a relative to sleeves 29 a-29 b for preventing the second brushes 30 a, 30 a from falling out of the respective sleeves 29 a-29 b by the spring forces of coil springs 32 a, 32 a.

An annular seal member 34 is interleaved between the outer periphery of the root (the basal end) of brush-retaining portion 28 a and an annular groove formed in the opening end of the cylindrical wall portion 3 c of cover main body 3 a.

As seen in FIG. 4, each of the diametrically-opposed tab-like brackets 28 c, 28 c is formed into a substantially triangular shape, and formed with a bolt insertion hole (a through hole) 28 e. Thus, brush retainer 28 is fixedly connected to the cover main body 3 a by means of bolts 4, 4 (see FIG. 2), which are inserted through the respective bolt insertion holes 28 e, 28 e of tab-like brackets 28 c, 28 c and screwed into respective female screw-threads (not shown) formed in the cover main body 3 a.

The previously-discussed motor output shaft 13 and eccentric shaft 39 are rotatably supported by means of the small-diameter ball bearing 37 and the needle bearing 38. Small-diameter ball bearing 37 is installed on the outer peripheral surface of the root of the shank 10 b near the head 10 a of cam bolt 10. On the other hand, needle bearing 38 is mounted on the outer peripheral surface of cylindrical portion 9 b of driven member 9, and arranged in close proximity to the right-hand side end (viewing FIG. 3) of small-diameter ball bearing 37 such that these bearings 37-38 are juxtaposed to each other.

Needle bearing 38 is comprised of a cylindrical retainer 38 a press-fitted into the inner peripheral surface of eccentric shaft 39 and a plurality of needle rollers 38 b (rolling elements) rotatably retained inside of the retainer 38 a. Each of needle rollers 38 b is in rolling-contact with the outer peripheral surface of cylindrical portion 9 b of driven member 9.

The inner ring of small-diameter ball bearing 37 is retained between the annular front end face of cylindrical portion 9 b of driven member 9 and the annular washer 10 c of cam bolt 10. On the other hand, the outer ring of small-diameter ball bearing 37 is press-fitted to the stepped portion defined between the small-inside-diameter section and the large-inside-diameter section of eccentric shaft 39, in a manner so as to be axially positioned in place by abutment with the inside annular face of the stepped portion of eccentric shaft 39.

A small-diameter oil seal (a seal member) 46 is interleaved between the outer peripheral surface of large-diameter portion 13 a of motor output shaft 13 (eccentric shaft 39) and the inner peripheral surface of axially-leftward extending cylindrical portion 5 c of housing 5, for preventing leakage of lubricating oil from the inside of speed reducer 8 toward the inside of electric motor 12.

The control unit (not shown) includes an input/output interface (I/O), memories (RAM, ROM), and a microprocessor or a central processing unit (CPU). The input/output interface (I/O) of the control unit receives input information from various engine/vehicle sensors, namely, a crank angle sensor, a cam shaft angle sensor, an airflow meter, an engine temperature sensor (an engine coolant temperature sensor), an accelerator opening sensor, and the like. Within the control unit, the CPU allows the access by the I/O interface of input informational data signals from the engine/vehicle sensors. The CPU is responsible for carrying the engine control program (i.e., the ignition-timing/throttle/fuel-injection/valve-timing control program) stored in memories, and is capable of performing necessary arithmetic and logic operations, depending on the current engine/vehicle operating condition, determined based on latest up-to-date informational data signals from the engine/vehicle sensors. Computational results (arithmetic calculation results), that is, calculated output signals are relayed through the output interface circuitry of the control unit to output stages (actuators), for electronic spark control, control of an electronically-controlled throttle valve, control of the fuel-injection system, and control of the VTC system. Concretely, the control unit is configured to detect an actual relative phase of intake camshaft 02 to sprocket 1 responsively to input informational signals from the crank angle sensor and the cam angle sensor and also configured to determine a desired relative phase of intake camshaft 02 to sprocket 1 depending on the current engine/vehicle operating condition. The control unit is further configured to perform rotational speed control of motor output shaft 13 by controlling electric-current supply to the electromagnetic coil 18 of electric motor 12. The rotational speed of motor output shaft 13 is reduced by means of the speed reducer 8. In this manner, the actual relative phase of intake camshaft 02 to sprocket 1 can be controlled and brought closer to the desired value.

As seen from the cross sections of FIGS. 3 and 5, and the perspective disassembled view of FIG. 4, speed reducer 8 is mainly comprised of the eccentric shaft 39 (constructing a part of the eccentric rotation member) that performs eccentric rotary motion, a middle-diameter ball bearing 47 (constructing the remainder of the eccentric rotation member) installed on the outer periphery of eccentric shaft 39, a plurality of rollers (serving as rolling elements) 48 rotatably installed on the outer periphery of middle-diameter ball bearing 47 and circumferentially arranged substantially at regular intervals, the cage 41 configured to partition, retain and guide these rollers 48, kept in rolling-contact with an outer ring 47 b (described later) of middle-diameter ball bearing 47, in the circumferential direction by respective roller-holding holes 41 b (in other words, respective axially-protruding lugs), while permitting a slight radial displacement (a slight oscillating motion) of each of rollers 48, and the driven member 9 formed integral with the cage 41, and the internal-tooth structural member 19 with the waveform internal toothed portion 19 a.

Eccentric shaft 39 is formed into a shouldered cylindrical-hollow shape. Eccentric shaft 39 is constructed by a small-diameter portion 39 a (at the front end) and a large-diameter portion 39 b (at the rear end). The small-diameter portion 39 a of eccentric shaft 39 is press-fitted into the inner peripheral surface of large-diameter portion 13 a of motor output shaft 13. The large-diameter portion 39 b of eccentric shaft 39 is a substantially cylindrical cam. The geometric center “Y” of the cam contour surface of the outer periphery of large-diameter portion 39 b of eccentric shaft 39 is slightly displaced from the axis “X” (i.e., the rotation center “X” shown in FIGS. 3 and 5) of motor output shaft 13 in the radial direction.

As viewed from the longitudinal cross section of FIG. 3, middle-diameter ball bearing 47 is comprised of an inner ring 47 a, the outer ring 47 b, and balls 47 c rotatably disposed and confined between them. The inner ring 47 a of ball bearing 47 is press-fitted onto the outer peripheral surface (i.e., the eccentric-cam contour surface) of large-diameter portion 39 b of eccentric shaft 39 in a manner so as to be axially positioned in place. In contrast to the inner ring 47 a, the outer ring 47 b is not securely fixed in the axial direction. That is, the outer ring 47 b is free and therefore is able to move contact-free. Concretely, the left-hand sidewall (viewing FIG. 3) of the outer ring 47 b, facing the electric-motor side, is kept out of contact with the housing 5 of electric motor 12, while the right-hand sidewall of the outer ring 47 b, axially opposed to the annular bottom of cage 41, is kept out of contact with the inside wall surface of the annular bottom of cage 41. More concretely, a very small axial clearance “Caxial” is defined between the right-hand sidewall of the outer ring 47 b and the inside wall surface of the annular bottom of cage 41, axially opposed to each other. Rollers 48, interleaved between the outer periphery of outer ring 47 b of middle-diameter ball bearing 47 and the waveform internal toothed portion 19 a of internal-tooth structural member 19, are held in rolling-contact with the outer peripheral surface of outer ring 47 b. A crescent-shaped annular clearance “Cannular” is defined between the outer peripheral surface of outer ring 47 b and the substantially comb-tooth shaped protruding portion (the substantially cylindrical portion 41 a) of cage 41. Owing to eccentric rotary motion of eccentric shaft 39, middle-diameter ball bearing 47 is radially moved or displaced by virtue of the crescent-shaped annular clearance “Cannular”. That is, the crescent-shaped annular clearance “Cannular” permits a slight radial displacement (a slight oscillating motion) of middle-diameter ball bearing 47.

Each of rollers 48 is made from iron-based metal material, and formed as a cylindrical solid roller. Owing to the eccentric displacement (oscillating motion) of middle-diameter ball bearing 47, the radially-inward contact surface of each of rollers 48, included within a given area, is brought into abutment (rolling-contact) with the outer peripheral surface of the outer ring 47 b of middle-diameter ball bearing 47. On the other hand, the radially-outward contact surfaces of some of rollers, associated with the given area, are fitted into some troughs of internal teeth 19 a of internal-tooth structural member 19 (serving as a toothed wheel or a toothed gear). That is, in the eccentric position of the eccentric rotation member (namely, the middle-diameter ball bearing 47 and eccentric shaft 39) shown in FIG. 5, roller 48, located at the 12 o'clock position, is brought into completely fitted-engagement (full tooth engagement) with the inner face of the trough between the uppermost two adjacent internal teeth 19 a, 19 a. In contrast, roller 48, located at the 6 o'clock position, is brought out of engagement. That is, owing to the eccentric displacement (oscillating motion) of the eccentric rotation member (i.e., the middle-diameter ball bearing 47 and eccentric shaft 39), rollers 48 can radially oscillate, while being circumferentially guided by respective axially-protruding lugs (respective roller-holding holes 41 b) of cage 41.

The phase converter 2 of intake VTC 04 is frequently operated over the entire range of engine speed and load after engine start-up by driving both the electric motor 12 and the speed reducer 8, so as to change or convert the angular phase of intake camshaft 02 relative to sprocket 1 (i.e., the engine crankshaft). That is, when the engine is started from cold, the relative phase of intake camshaft 02 is controlled to a given intermediate angular phase between the maximum phase-advance position and the maximum phase-retard position, thereby ensuring a better engine startability. When the engine operating condition changes between low-speed and low-load operation and high-speed and high-load operation, a phase change of intake camshaft 02 between the maximum phase-retard position and the maximum phase-advance position is repeatedly executed, thereby reducing a pumping loss and consequently ensuring improved fuel economy and enhanced engine power output. Furthermore, it is possible to improve the exhaust emission performance in combination with operation of exhaust VTC 05 (described later).

Therefore, as seen from the characteristic diagram of FIG. 11, an electric motor having a drive-efficiency characteristic such that the motor drive efficiency (a positive efficiency η) is set to become higher in a comparatively high motor-speed range is used for the intake-VTC-side electric motor 12.

Additionally, the intake-VTC-side speed reducer 8 is constructed by a cycloid speed reducer that uses a plurality of rollers 48. Note that, in the valve timing control system of the embodiment, the reduction ratio of the intake-VTC-side speed reducer 8 of intake VTC 04 is set to be greater than that of the exhaust-VTC-side speed reducer 8 (described later) of exhaust VTC 05.

To ensure smooth operation of the electric-motor-driven phase-converter equipped VTC apparatus, lubricating oil is supplied into the internal space of speed reducer 8 by lubricating-oil supply means. As shown in FIG. 3, the lubricating-oil supply means is comprised of an annular oil supply passage (not numbered), which is annularly grooved in the outer periphery of the journal of intake camshaft 02 rotatably supported by camshaft-journal bearing member 06 mounted on the cylinder head 01 and to which lubricating oil is supplied from a main oil gallery (not shown), an axial oil supply hole 51, a small-diameter axial oil hole 52, and large-diameter oil drain holes (not shown). Axial oil supply hole 51 is formed in the front end of intake camshaft 02 to communicate the annular oil supply passage via an oil groove, cut in the front end face of intake camshaft 02 and configured to communicate the downstream end of axial oil supply hole 51. Small-diameter axial oil hole 52 is formed as a through hole in the driven member 9, such that one end of small-diameter axial oil hole 52 is opened into the axial oil supply hole 51 through the oil groove cut in the camshaft end face and the other end of small-diameter axial oil hole 52 is opened into the internal space defined near both the needle bearing 38 and the middle-diameter ball bearing 47. Large-diameter oil drain holes (not shown) are formed in the driven member 9 as oil outlets.

During operation, lubricating oil is constantly fed from the discharge port of an oil pump (not shown) into the oil supply hole 51 via the main oil gallery formed in the cylinder head. Hence, by the previously-discussed lubricating-oil supply means, lubricating oil can be fed via the oil supply hole 51 to the internal space 44 and stays in the internal space 44. Then, the lubricating oil is supplied from the internal space 44 to moving parts, namely, middle-diameter ball bearing 47 and rollers 48 for lubrication, and further flows into the eccentric shaft 39 and the internal space of motor output shaft 13, for lubrication of moving parts, such as needle bearing 38 and small-diameter ball bearing 37. By the way, undesirable leakage of lubricating oil, staying in the internal space 44, to the inside of the electric-motor housing 5 can be prevented or adequately suppressed by means of the small-diameter oil seal 46.

The fundamental operation of intake VTC 04 incorporated in the VTC system of the embodiment is hereunder described in detail.

When the engine crankshaft rotates, sprocket 1 rotates in synchronism with rotation of the crankshaft through the timing chain (not shown). On one hand, torque flows from the sprocket 1 through the internal-tooth structural member 19 via the annular female screw-threaded member 6 to the housing 5 of electric motor 12, and thus permanent magnets 14-15 and stator 16, all attached to the inner periphery of housing 5, rotate together with the housing 5. On the other hand, torque flows from the sprocket 1 through the internal-tooth structural member 19 via the rollers 48, cage 41, and driven member 9 to the intake camshaft 02. Thus, intake camshaft 02 is rotated to operate (open/close) the intake valves against the spring forces of the valve springs by the intake-valve cams.

During a given engine operating condition after the engine start-up, an electric current is applied from the control unit through the terminal strips 31, 31, pig-tale harnesses 33, 33, second brushes 30 a, 30 a, and slip rings 26 a-26 b to the electromagnetic coil 18 of electric motor 12. Hence, motor output shaft 13 is driven. Then, the output rotation from the motor output shaft 13 is reduced by means of the speed reducer 8, and thus the reduced motor speed (in other words, the multiplied motor torque) is transmitted to the intake camshaft 02.

That is, when eccentric shaft 39 rotates eccentrically during rotation of motor output shaft 13, each of rollers 48 moves (rolls) and relocates from one of two adjacent internal teeth 19 a, 19 a to the other with one-tooth displacement per one complete revolution of motor output shaft 13, while being held in rolling-contact with the outer ring 47 b of middle-diameter ball bearing 47 and simultaneously radially guided by the associated axially-protruding lug (the associated roller-holding hole 41 b) of cage 41. By way of the repeated relocations of each of rollers 48 every revolutions of motor output shaft 13, rollers 48 move in the circumferential direction with respect to the waveform internal toothed portion 19 a of internal-tooth structural member 19, while being held in rolling-contact with the outer ring 47 b of middle-diameter ball bearing 47. In this manner, torque is transmitted through the driven member 9 to the intake camshaft 02, while the rotational speed of motor output shaft 13 is reduced.

As discussed above, by execution of rotational speed control of motor output shaft 13, intake camshaft 02 is rotated in a normal-rotational direction or in a reverse-rotational direction with respect to the sprocket 1, and thus an angular phase of intake camshaft 02 relative to sprocket 1 is changed, and as a result intake valve open timing (IVO) and intake valve closure timing (IVC) can be phase-advanced or phase-retarded.

The reduction ratio of this type of speed reducer 8 can be determined by the number of rollers 48, in other words, the number of roller-holding holes 41 b (i.e., the number of axially-protruding lugs of cage 41). The fewer the number of rollers 48, the lower the reduction ratio. That is, the reduction ratio can be arbitrarily set depending on the number of rollers 48.

As discussed above, the speed reducer 8, incorporated in the intake VTC 04, is configured such that the rotational speed of motor output shaft 13 of electric motor 12 can be reduced by virtue of the repeated relocations of each of rollers 48 every revolutions of motor output shaft 13, rollers 48 moving in the circumferential direction with respect to the waveform internal toothed portion 19 a of internal-tooth structural member 19, while being held in rolling-contact with the outer ring 47 b of middle-diameter ball bearing 47. Hence, a friction of intake VTC 04 during operation (in other words, during speed-reduction of the speed reducer 8) becomes adequately reduced. Thus, it is possible to enhance or improve the phase-conversion responsiveness for the angular phase shift of intake camshaft 02 relative to sprocket 1 in the phase-advance direction or in the phase-retard direction.

[Exhaust VTC]

As shown in FIGS. 1 and 8-10, regarding the electric-motor-driven exhaust VTC 05, its fundamental configuration is identical to the electric-motor-driven intake VTC 04. Thus, in explaining the exhaust VTC 05, for the purpose of simplification of the disclosure, the same reference signs used to designate elements in the intake VTC 04 will be applied to the corresponding elements used in the exhaust VTC 05, while detailed description of the same reference signs will be omitted because the above description thereon seems to be self-explanatory.

The exhaust VTC 05 differs from the intake VTC 04 in that a biasing mechanism 70 is provided between the exhaust camshaft 03 and the driven member 9, for biasing or forcing the exhaust camshaft 03 in the phase-advance direction relatively to the sprocket 1, and that a drive torque of electric motor 12 and a reduction ratio of speed reducer 8 in the exhaust VTC 05 differ from those in the intake VTC 04.

That is to say, as shown in FIG. 8, exhaust VTC 05 is comprised of a sprocket 1 (serving as a driving rotary member) that rotates in synchronism with rotation of the engine crankshaft, and a phase change mechanism (a phase converter) 2 installed between the sprocket 1 and the exhaust camshaft 03 for changing a relative angular phase between the sprocket 1 and the exhaust camshaft 03 depending on an engine operating condition.

Sprocket 1 is comprised of a cylindrical sprocket body 1 a, a timing gear 1 b formed integral with the outer periphery of sprocket body 1 a, and an internal-tooth structural member 19. Sprocket body 1 a is made from iron-based metal material, and formed integral with the timing gear 1 b. Timing gear 1 b receives torque from the crankshaft through the common timing chain (not shown) wound on the sprocket on the crankshaft, the sprocket 1 on the intake camshaft side and the sprocket 1 on the exhaust camshaft side. Internal-tooth structural member 19 is formed integral with the front end of sprocket body 1 a on the exhaust camshaft side.

The sprocket body 1 a shown in FIG. 8 and partly constructing the exhaust-VTC-side internal-tooth structural member 19 is configured to be axially longer than the sprocket body 1 a shown in FIG. 3 and partly constructing the intake-VTC-side internal-tooth structural member 19.

Biasing mechanism 70 is mainly comprised of a spring retainer 71 and a torsion spring 72. Spring retainer 71 is installed between the front end of exhaust camshaft 03 and the fixed-end portion 9 a of driven member 9. The driven member 9, the spring retainer 71, and the front end of exhaust camshaft 03 are integrally connected to each other by screwing the cam bolt 10 into the axial end of exhaust camshaft 03. Torsion spring 72 is installed on the outer periphery of spring retainer 71.

Spring retainer 71 is shaped into a substantially cylindrical shape having a short axial length. Spring retainer 71 has a central bolt insertion hole (a through hole) 71 a into which the shank 10 b of cam bolt 10 is inserted. On one hand, the central portion of the front end of spring retainer 71 is formed with a cylindrical fitted bore 71 b into which a cylindrical-hollow axially-protruding portion 9 c extending backward from the central portion of the disk-shaped fixed-end portion 9 a of driven member 9 is fitted. On the other hand, the central portion of the rear end of spring retainer 71, facing apart from the cylindrical fitted bore 71 b, is formed with a cylindrical-hollow axially-protruding portion 71 c is fitted into a cylindrical fitted bore 03 a formed in the center of the front end of exhaust camshaft 03.

Additionally, spring retainer 71 has a communication hole (an axial through hole) 71 d through which the axial oil supply hole 51 formed in the front end of exhaust camshaft 03 and the axial oil hole 52 formed in the driven member 9 are communicated with each other. Also, the outer periphery of the rear end of spring retainer 71, facing the front end of exhaust camshaft 03, is partly cut or machined as a first spring-hook retaining slit 71 e with which one end (a first hook, described later) 72 a of torsion spring 72 is brought into engagement in the radial direction.

As best seen in FIG. 9, torsion spring 72 is loosely assembled or fitted over the outer periphery of spring retainer 71, in such a manner as to permit torsion spring 72 to diametrically expand or contract without tightly fitting the inside diameter of torsion spring 72 over the outer periphery of spring retainer 71. When assembling, the one end (the first short torsion-spring hook) 72 a, bent radially inward, is brought into engagement with the first spring-hook retaining slit 71 e of spring retainer 71. The other end (a second short torsion-spring hook) 72 b, bent radially outward, is brought into engagement with a second spring-hook retaining slit 1 d, which slit is formed in the timing gear 1 b of sprocket body 1 a, in the radial direction.

By virtue of the installed torsion spring 72, exhaust camshaft 03 is permanently biased or forced in the phase-advance direction indicated by the arrow in FIG. 9 relatively to the sprocket 1. Hence, during the engine starting period, there is no valve overlapping period where exhaust and intake valves are both open. This contributes to the better combustibility and improved startability.

By the way, a cylindrical pressing member 73 is installed on the inner periphery of sprocket body 1 a, for axially supporting the outer ring 43 a of large-diameter ball bearing 43 through the retainer plate 61, while applying a slight push from the inside face of retainer plate 61, caused by axial forces of bolts 7, to the rearward end face 43 e of the outer ring 43 a.

As appreciated from the longitudinal cross section of FIG. 8, a locate pin 74 is provided or axially press-fitted, for the purpose of radial positioning of exhaust camshaft 03 and spring retainer 71. In a similar manner, a locate pin 75 is provided or axially press-fitted, for the purpose of radial positioning of spring retainer 71 and driven member 9.

In contrast to the intake VTC 04, in the case of the exhaust VTC 05, a phase-change (a phase-conversion) in relative-rotation phase of exhaust camshaft 03 with respect to sprocket 1 is not frequently executed during operation of the engine. For instance, during the engine starting period, the phase of exhaust camshaft 03 relative to sprocket 1 is forcibly changed or converted in the phase-advance direction by the spring force of torsion spring 72 without driving the electric motor 12 of exhaust VTC 05. When the engine operating condition changes between middle-speed operation and high-speed operation, the phase of exhaust camshaft 03 relative to sprocket 1 is held at a substantially intermediate phase-angle position. Also, when the engine is running at low speeds, the electric motor 12 of exhaust VTC 05 is often driven.

For the reasons discussed above, as appreciated from the motor-efficiency diagram of FIG. 11, the motor drive efficiency η of the exhaust-VTC-side electric motor 12 is set to become maximum in a low motor-speed range. As previously described, the motor drive efficiency η of the intake-VTC-side electric motor 12 is set to become maximum in a high motor-speed range. That is, the maximum drive-efficiency range of the exhaust-VTC-side electric motor 12 is set to be different from that of the intake-VTC-side electric motor 12.

On the other hand, as appreciated from comparison between the cross sections of FIGS. 5 and 10, the number of internal teeth 19 a of internal-tooth structural member 19 of the exhaust-VTC-side speed reducer 8 is set to be fewer than that of the intake-VTC-side speed reducer 8. That is, depending on the fewer internal teeth 19 a of the exhaust-VTC-side speed reducer 8, the number of rollers 48 of the exhaust-VTC-side speed reducer 8 is decreased. Hence, the reduction ratio of the exhaust-VTC-side speed reducer 8 is set to be less than that of the intake-VTC-side speed reducer 8.

Additionally, regarding the electricity-feeding device of exhaust VTC 05, synthetic-resin plate 22, synthetic-resin brush holders 23 a-23 b, and brush retainer 28 are made from nylon resin materials.

As will be appreciated from the above, the drive-efficiency characteristics of the intake-VTC-side electric motor 12 and the exhaust-VTC-side electric motor 12 are set independently of each other, such that the respective motor drive efficiencies become high depending on two different motor-speed ranges, namely a frequently-used motor-speed range suitable for the intake-VTC-side electric motor and a frequently-used motor-speed range suitable for the exhaust-VTC-side electric motor. Thus, it is possible to more efficiently drive each of intake-VTC-side electric motor and the exhaust-VTC-side electric motor.

Accordingly, when switching, shifting, or changing an angular phase (i.e., intake valve open timing and intake valve closure timing) of intake camshaft 02 relative to the sprocket 1 on the side of intake VTC 04 and an angular phase (i.e., exhaust valve open timing and exhaust valve closure timing) of exhaust camshaft 03 relative to the sprocket 1 on the side of exhaust VTC 05, it is possible to improve both the operational responsiveness of intake VTC 04 for the phase shift of intake camshaft 02 relative to the intake-side sprocket and the operational responsiveness of exhaust VTC 05 for the phase shift of exhaust camshaft 03 relative to the exhaust-side sprocket, thereby exhibiting a sufficient engine performance.

Furthermore, as discussed above, it is possible to more efficiently drive both the electric motor 12 on the side of intake VTC 04 and the electric motor 12 on the side of exhaust VTC 05, and hence it is possible to reduce the drive load on each of the intake-VTC-side electric motor 12 and the exhaust-VTC-side electric motor 12. This contributes to the enhanced durability of each of the intake-VTC-side electric motor 12 and the exhaust-VTC-side electric motor 12.

Moreover, in the valve timing control system of the embodiment, the speed reducer 8 on the side of intake VTC 04 and the speed reducer 8 on the side of exhaust VTC 05 are configured, such that the reduction ratio of the intake-VTC-side speed reducer 8 and the reduction ratio of the exhaust-VTC-side speed reducer 8 differ from each other. That is, the reduction ratio of the intake-VTC-side speed reducer 8 in the intake VTC 04, by means of which intake-valve timing can be frequently switched or changed over the entire engine operating range, is set to be greater than the reduction ratio of the exhaust-VTC-side speed reducer 8 in the exhaust VTC 05. Thus, it is possible to remarkably enhance the operational responsiveness of intake VTC 04. Hence, in combination with the previously-discussed appropriate setting of the drive efficiency η of the intake-VTC-side electric motor 12 in which the motor drive efficiency η becomes maximum in a high motor-speed range, it is possible to further enhance the operational responsiveness of intake VTC 04, and consequently more greatly improve the intake-valve-timing control responsiveness.

As discussed previously, regarding the exhaust VTC 05, by decreasing the number of internal teeth 19 a of internal-tooth structural member 19 of the exhaust-VTC-side speed reducer 8, in other words, by decreasing the number of rollers 48 of the exhaust-VTC-side speed reducer 8, the reduction ratio of the exhaust-VTC-side speed reducer 8 is set to be less than that of the intake-VTC-side speed reducer 8. In comparison with the intake-VTC-side speed reducer 8, the decreased number of internal teeth 19 a (i.e., the decreased number of rollers 48) in the exhaust-VTC-side speed reducer 8 contributes to the reduced manufacturing costs and improved assembling efficiency.

Additionally, regarding the intake-VTC-side electricity-feeding device that requires a very high heat resistance owing to a comparatively long total operation time of the intake-VTC-side electric motor 12, synthetic-resin plate 22, synthetic-resin brush holders 23 a-23 b, and brush retainer 28 are made from PPS (polyphenylene sulfide) synthetic resin materials having a high heat-resisting property. In contrast, regarding the exhaust-VTC-side electricity-feeding device that does not require a very high heat resistance owing to a comparatively short total operation time of the exhaust-VTC-side electric motor 12, synthetic-resin plate 22, synthetic-resin brush holders 23 a-23 b, and brush retainer 28 are made from inexpensive nylon resin materials. This also contributes to the reduced manufacturing costs in the exhaust VTC 05.

As will be appreciated from the above, the invention is not limited to the particular embodiments shown and described herein, but various changes and modifications may be made. For instance, the drive-efficiency settings of the intake-VTC-side electric motor 12 and the exhaust-VTC-side electric motor 12 and the reduction-ratio settings of the intake-VTC-side speed reducer 8 and the exhaust-VTC-side speed reducer 8 may be appropriately modified or arbitrarily set depending on the type of the internal combustion engine and the specification and size of each of intake VTC 04 and exhaust VTC 05.

As a matter of course, the type of electric motor 12 and the type of speed reducer 8 are not limited to the particular embodiment shown and described herein. In the shown embodiment, a brush-equipped direct-current motor is used for the electric motor 12. In lieu thereof, a brushless motor may be used for the electric motor 12. Also, in the shown embodiment, a roller-type cycloid speed reducer employing a plurality of rollers 48 is used for the speed reducer 8. In lieu thereof, the speed reducer 8 may be constructed by a planetary-gear type cycloid speed reducer such as disclosed in U.S. Pat. No. 7,959,537 issued Jun. 14, 2011, the teachings of which are hereby incorporated by reference.

The entire contents of Japanese Patent Application No. 2013-040325 (filed Mar. 1, 2013) are incorporated herein by reference.

While the foregoing is a description of the preferred embodiments carried out the invention, it will be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the scope or spirit of this invention as defined by the following claims. 

What is claimed is:
 1. A valve timing control system of an internal combustion engine, comprising: an electric-motor-driven intake valve timing control device installed on an intake camshaft and having a first electric motor provided to generate torque by energizing the first electric motor; and an electric-motor-driven exhaust valve timing control device installed on an exhaust camshaft and having a second electric motor provided to generate torque by energizing the second electric motor, wherein a maximum drive-efficiency motor-speed range of the first electric motor of the electric-motor-driven intake valve timing control device and a maximum drive-efficiency motor-speed range of the second electric motor of the electric-motor-driven exhaust valve timing control device are set to differ from each other depending on a first working range suitable for the electric-motor-driven intake valve timing control device and a second working range suitable for the electric-motor-driven exhaust valve timing control device.
 2. The valve timing control system as recited in claim 1, wherein: the maximum drive-efficiency motor-speed range of the first electric motor of the electric-motor-driven intake valve timing control device, whose working range is wider than the working range of the electric-motor-driven exhaust valve timing control device, is set higher than that of the second electric motor of the electric-motor-driven exhaust valve timing control device.
 3. The valve timing control system as recited in claim 1, wherein: each of the first electric motor of the electric-motor-driven intake valve timing control device and the second electric motor of the electric-motor-driven exhaust valve timing control device is a brush-equipped direct-current motor.
 4. A valve timing control system of an internal combustion engine, comprising: an electric-motor-driven intake valve timing control device installed on an intake camshaft, the electric-motor-driven intake valve timing control device comprising: a first electric motor provided to generate torque by energizing the first electric motor; and a first speed reducer configured to reduce a rotational speed of the first electric motor, and transmit the reduced rotational speed to the intake camshaft for changing intake valve timing; and an electric-motor-driven exhaust valve timing control device installed on an exhaust camshaft, the electric-motor-driven exhaust valve timing control device comprising: a second electric motor provided to generate torque by energizing the second electric motor; and a second speed reducer configured to reduce a rotational speed of the second electric motor, and transmit the reduced rotational speed to the exhaust camshaft for changing exhaust valve timing, wherein a reduction ratio of the first speed reducer for a reduction in the rotational speed of the first electric motor and a reduction ratio of the second speed reducer for a reduction in the rotational speed of the second electric motor are set to differ from each other.
 5. The valve timing control system as recited in claim 4, wherein: the reduction ratio of the first speed reducer of the electric-motor-driven intake valve timing control device is set to be greater than the reduction ratio of the second speed reducer of the electric-motor-driven exhaust valve timing control device.
 6. The valve timing control system as recited in claim 4, wherein: each of the first speed reducer of the electric-motor-driven intake valve timing control device and the second speed reducer of the electric-motor-driven exhaust valve timing control device is a cycloid speed reducer employing a plurality of rollers.
 7. A valve timing control system of an internal combustion engine, comprising: an electric-actuator equipped intake valve timing control device installed on an intake camshaft, the electric-actuator equipped intake valve timing control device comprising: a first electric actuator provided to generate torque by energizing the first electric actuator; and a first speed reducer configured to reduce a rotational speed of the first electric actuator, and transmit the reduced rotational speed to the intake camshaft for changing intake valve timing; and an electric-actuator equipped exhaust valve timing control device installed on an exhaust camshaft, the electric-actuator equipped exhaust valve timing control device comprising: a second electric actuator provided to generate torque by energizing the second electric actuator; and a second speed reducer configured to reduce a rotational speed of the second electric actuator, and transmit the reduced rotational speed to the exhaust camshaft for changing exhaust valve timing, wherein a reduction ratio of the first speed reducer for a reduction in the rotational speed of the first electric actuator and a reduction ratio of the second speed reducer for a reduction in the rotational speed of the second electric actuator are set to differ from each other. 